The light water nuclear reactors typically comprise a pressure containment vessel containing a core constituted of vertical fuel assemblies located in side-by-side relation, each having fuel elements distributed at the nodes of a regular network, each fuel element containing fissile material and possibly fertile material. The fuel elements are substituted at some of the nodes of the network with guide tubes slidably receiving movable rods of control clusters. The fuel assemblies are at least partially substituted with other fuel assemblies after each burn-up cycle of the reactor.
In each fuel assembly, the fuel elements are separated by a gap which flows cooling and moderating water. A moderation ratio VM/VU is defined as a ratio of a moderator module VM to the fissile material volume VU in the core.
Conventional light water reactors now in operation have a moderation ratio such that the energy spectrum of the neutrons is thermal. Two directions have been explored for improving the light water reactors and for a better use of the fuel material. Both directions imply that the moderation ratio is at least temporarily decreased.
The first approach consists of varying the neutron energy spectrum as the fuel burns up during a cycle. The natural uranium consumption and the initial degree of enrichment may be decreased for a predetermined burn-up rate. A spectral shift reactor which appears of particular interest is described in French Patent Application No. 82 18011 (FR-A-2,535,509). That reactor is comparable in structure to the conventional PWRs but includes a mechanical device for shifting the neutron spectrum, comprising clusters of rods containing fertile material, such as natural or depleted uranium oxide. The clusters are movable for insertion into the core or removal from the core during operation of the reactor. When fertile rods are introduced in guide tubes of the assemblies, they force moderating water out of the guide tubes and decrease the moderator volume VM in the core. As a consequence, during an operating cycle of the core, the neutron energy spectrum may be shifted. During a first part of the cycle, the clusters of fertile rods are maintained in the core. They shift the energy spectrum toward higher energies and increase the conversion rate of fertile material (uranium 238) into fissile material (plutonium). During a second part of the cycle, the clusters dedicated to spectral shift are progressively removed. The fissile material formed during the first part of the cycle is then partially burnt. The conversion rate is increased by about 10% with respect to a conventional thermal neutron PWR due to conversion rate increase.
The other approach consists of under-moderating the reactor at all times. Then it is possible to use a mixed fuel comprising natural uranium and plutonium with a "breeding" rate of plutonium of about 1. However, the necessary decrease of the moderation rate VM/VU typically requires that the fuel elements be located in the fuel assemblies in a triangular rather than square array.
Most under-moderated reactors comprise two types of fuel assemblies. Some, called fissile assemblies, contain principally fissile material; the others, called fertile assemblies, contain a material capable of being converted to fissile material under the effect of neutron bombardment. The fertile assemblies are generally disposed at the periphery of the core where they collect neutrons produced by the fissile assemblies.
Proposals have been made for combining the advantages of spectral shift (particularly the gain on uranium consumption) and of under-moderation (plutonium breeding rate possibly greater than 1, possible use of depleted uranium, increased cycle duration). French Patent Application No. 83 15591 discloses a reactor having a heterogeneous core in which fuel assemblies of a type suitable for use in under-moderated reactors and fuel asssemblies for spectral shift reactors are associated for best utilization of plutonium previously produced in the fuel assemblies of conventional thermal neutron PWRs. That plutonium is recovered during reprocessing of spent fuel assemblies.
However, that approach requires that a utility which has a plurality of nuclear power plants should dedicate at least one power plant to use of recovered plutonium.
A first consideration suggests that the difficulty cannot be overcome since that type of fuel assembly which is required for under-moderated reactors does not lend to use in association with the internals and the control system of a reactor for operation with thermal neutrons, and conversely.